Documents U.S. Pat. No. 5,259,120/EP0526056 or U.S. Pat. No. 5,813,128/EP0786644 disclose a device, generally known as a “ball-bar” device, and a method for analyzing the precision with which a machine tool follows a path. That device of the prior art consists in a telescopic bar connected by balls at its ends between a fixed point on one side and a mobile point on the other side, carried physically by a part of a machine tool, generally the spindle of said machine, which is adapted to move in relation to said fixed point. The method consists in moving the mobile point along a circular path centered on the fixed point and measuring, during that path, the length variations of the telescopic bar. Those length variations correspond to path following faults, and make it possible to characterize the dynamic precision of the machine tool being tested. In the case of a machine tool, the machine is highly rigid in view of the forces to which said machine is subjected during use, so that such dynamic characterization is representative of the precision of the machine. In the case of a robot, the kinematic chain is much less rigid than that of a machine tool, so that such characterization is not representative of the capacities of said robot, and the precision and ability of said robot to follow a path are affected by the loads applied to it. In the prior art, a robot is thus characterized statically vis-a-vis the stiffness of its kinematic chain by applying, through a loading device, a force screw to its part adapted to receive an effector, generally the wrist, while the movements are blocked and by measuring the deviation of the position of said wrist under the effect of said screw. These measurements characterize the stiffness of the kinematic chain and bring out the play, play under load and positioning hysteresis, which data are introduced in control correction tables that make it possible to compensate for such faults in operation, at least in part, essentially to obtain precise positioning. This method of the prior art is sufficient for carrying out manipulation operations known as “pick and place” or generally fixed positioning operations such as drilling a hole or welding a tack. However, robots are increasingly used for operations that make it necessary to follow a path, such as trimming, deburring, orbital drilling, bead welding, friction stir welding or additive fabrication, without this list being exhaustive; said path must be followed at speed that is fully controlled. The characterization methods of the prior art do not make it possible to understand and then correct the combined effects of accelerations and forces on the quality of the following of said path.
Document U.S. Pat. No. 6,662,261 describes a device for calibrating the movements of a robot or a machine tool along 6 degrees of freedom, using 6 telescopic measuring bars that extend between 2 assemblies of 3 spheres each contained in a plane, where one is fixed and the other is related the mobile part of the machine or robot. In a particular embodiment of the device, the 6 measuring bars are active and make it possible to apply a force screw on the plane defined by the 3 spheres linked to the mobile part. However, the device is complex, in terms of its design, its implementation and also the interpretation of the results derived from it.